Angular rate sensor with suppressed linear acceleration response

ABSTRACT

An angular rate sensor having two generally planar proof masses disposed along a drive axis in the plane of the masses, a sense axis perpendicular to the drive axis, and an input axis perpendicular to the plane of the masses. The masses are suspended from a pair of driving frames, which are mounted and constrained for anti-phase linear movement along the drive axis in drive-mode. Detectors responsive to the anti-phase movement of the masses in directions parallel to the sense axis in response to Coriolis forces produced by rotation of the masses about the input axis for monitoring rate of rotation about the input axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 12/692,606, filed on Jan. 23,2010, now pending.

BACKGROUND OF INVENTION

1. Field of Invention

This invention pertains generally to micromachined angular rate sensorsor gyroscopes, and more particularly to vibratory rate sensors utilizingdual-mass anti-phase translational movement in drive or sense modes.

2. Related Art

Many applications in automotive and consumer market require rate sensorsto be insensitive to linear acceleration in all directions. In general,a rate sensor can be susceptible to linear acceleration in the drive andsense directions of the proof masses of the sensor.

For rate sensors utilizing translational drive motion, commonly useddual-mass approach with conventional coupling method, such as thatdisclosed in U.S. Pat. No. 5,895,850, has an in-phase linear vibrationmode with resonant frequency below that of the anti-phase drive-mode.This parasitic mode allows the centroid of the proof masses to move inresponse to linear acceleration along the drive axis, so that thevelocities of the masses are susceptible to linear acceleration in thedrive direction. To reduce this susceptibility, the in-phase mode mustbe well suppressed such that its resonant frequency is much higher thanthat of anti-phase drive-mode.

Geen disclosed a mechanical coupling method in U.S. Pat. No. 5,635,638,which allows anti-phase linear movement of the two-coupled masses butresists their in-phase movement. However, the in-phase mode cannot bewell suppressed due to the fact that the arcuate coupling member used isprone to bending and buckling under linear acceleration forces in thedrive direction. Furthermore, no actual gyroscope device utilizing thatcoupling method has been disclosed.

Geen disclosed a different mechanical coupling method in U.S. Pat. No.6,877,374 in a Z-axis gyroscope that leads to a better suppression ofin-phase translational movement of the proof masses. However, thiscoupling structure is complex which inludes multiple levers, pivots, andflexures, and each proof mass has to split into two parts that move inarcuate motion, instead of pure linear movement. The unbalanced arcuatemotion of the two parts of a proof mass due to the process imperfectionsresults in an undesired net movement of the masse in sense direction,which causes the error of the sensor.

U.S. Pat. No. 7,036,372 discloses a Z-axis gyroscope that utilizes amechanical linkage between two masses which resists in-phase movement.Again, that linkage is complicated with requirement of multiple stiffrotation beams with inner anchor points as pivots that occupy aconsiderable layout area. Furthermore, the linkage must include pivotsanchored to the substrate. That makes it impossible to mount the linkageon movable structure, which is often desirable in multi-axes ratesensors.

For many vibratory dual-mass gyroscopes, the rejection of linearacceleration in sense direction, which is perpendicular to drive axis,is achieved by differential processing of the electrical signals fromthe sense responses of the two masses. The spurious sense signals causedby linear acceleration noise can only be cancelled when the two massesand their resonant frequencies of sense modes are identical, which areimpractical due to the process imperfections. And the un-cancelledresidual signal becomes the error and noise in the rate signal.

OBJECTS AND SUMMARY OF THE INVENTION

It is in general an object of the invention to provide a new andimproved angular rate sensor or gyroscope.

Another object of the invention is to provide a rate sensor or gyroscopeof the above character, which overcomes the limitations, anddisadvantages of rate sensors of the prior art.

These and other objects are achieved in accordance with the invention byproviding an angular rate sensor having two generally planar proofmasses, a planar supporting frame, means for suspending the masses fromthe supporting frame for movement along a sense axis in the plane of themasses, an input axis in the plane of the masses perpendicular to thesense axis, means for driving the masses and the supporting frame tooscillate about the input axis in drive-mode, means for constraining themasses for anti-phase sense-mode oscillation along the sense axis inresponse to Coriolis forces produced by rotation of the masses about theinput axis, means responsive to the anti-phase sense-mode movement ofthe masses for monitoring rate of rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of an embodiment of a y-axis rate sensoraccording to the present invention.

FIG. 2( a) is an operational view, illustrating the anti-phasedrive-mode movement of the embodiment of FIG. 1.

FIG. 2( b) is an operational view, illustrating the in-phase modemovement of the embodiment of FIG. 1.

FIG. 3 is a top plan view of another embodiment of a y-axis rate sensoraccording to the present invention.

FIG. 4 is a top plan view of an embodiment of a z-axis rate sensoraccording to the present invention.

FIG. 5 is a top plan view of another embodiment of a z-axis rate sensoraccording to the present invention.

FIG. 6 is a top plan view of another embodiment of a y-axis rate sensoraccording to the present invention.

FIG. 7 is a top plan view of an embodiment of a dual-axis rate sensoraccording to the present invention for detecting rate of rotation aboutthe y-, and z-axes.

FIG. 8 is a top plan view of another embodiment of a z-axis rate sensoraccording to the present invention.

FIG. 9 is a top plan view of another embodiment of a z-axis rate sensoraccording to the present invention.

FIG. 10 is a top plan view of another embodiment of a z-axis rate sensoraccording to the present invention.

FIG. 11 is a top plan view of another embodiment of a y-axis rate sensoraccording to the present invention.

FIG. 12 is a top plan view of an embodiment of a dual-axis rate sensoraccording to the present invention for detecting rate of rotation aboutthe x-, and y-axes.

FIG. 13 is a top plan view of another embodiment of a z-axis rate sensoraccording to the present invention.

FIG. 14 is a top plan view of another embodiment of a z-axis rate sensoraccording to the present invention.

FIG. 15 is a top plan view of an embodiment of a dual-axis rate sensoraccording to the present invention for detecting rate of rotation aboutthe x-, and z-axes.

FIG. 16 is a top plan view of an embodiment of a tri-axis rate sensoraccording to the present invention for detecting rate of rotation aboutthe x-, y-, and z-axes.

FIG. 17 is a top plan view of another embodiment of a tri-axis ratesensor according to the present invention for detecting rate of rotationabout the x-, y-, and z-axes.

DETAILED DESCRIPTION

As illustrated in FIG. 1, the rate sensor has a pair of generally planarproof masses 20 a, 20 b that lie in an x, y reference plane when thedevice is at rest. The masses are disposed and spaced apart along thex-axis, which is the drive axis, i.e. the axis along which the massesare driven to oscillate in drive-mode. The input axis, i.e. the axisabout which the angular rate of rotation is measured, is the y-axis.

The proof masses are suspended from a supporting frame, or sensingframe, 22 in the plane of the masses by flexible beams, or flexures, 24a, 24 b. The flexures, which extend along the y-axis, are relativelyflexible in the drive direction, but relatively stiff in otherdirections. These flexures constrain each one of the masses for linearmovement along the x-axis.

The masses can be driven into oscillation along the drive axis by combdrives 44 a, 44 b, which have interdigitated comb fingers with moveablefingers attached to masses 20 a, 20 b and stationary fingers affixed tothe anchored electrodes 46 a, 46 b respectively.

The sensing frame is generally planner and disposed in the x, y planewith and surrounding the masses. It is suspended from substrate (notshown) by a pair of torsion springs 26, which are spaced apart andextend along the y-axis and connect to anchors 28 affixed to thesubstrate. These torsion springs constrain the sensing frame forrotation about the y-axis in response to the Coriolis forces produced bythe rotation of the masses about the input axis, i.e. the y-axis, whenthe masses are driven to oscillate in anti-phase manner along thex-axis, as indicated by the arrows 21 a, 21 b.

The torsion movement of the sensing frame and the masses about they-axis in sense-mode is detected capacitively by a pair of capacitorsformed between masses 20 a, 20 b and corresponding electrode plates 42a, 42 b, which are mounted under the masses respectively on thesubstrate. These capacitors work together to serve as a differentialsensor, or detector, for monitoring the rate of rotation about they-axis.

A pair of mechanical linkages, or anti-phase couplers, 30, which aremounted on the sensing frame and connected between the masses, couplethe masses together and constrain them for precise anti-phase movementin drive-mode. The two couplers are spaced apart along the y-axis anddisposed symmetrically about the x-axis.

Each anti-phase coupler includes a supporting beam 34 and a pair of linksprings 32 a, 32 b. The supporting beam extends along the x-axis and isaffixed to the sensing frame at its two ends. The link springs extendfrom respective masses to the supporting beam. They are tilted slightlyfrom the y-axis and disposed symmetrically about that axis. The two linksprings join together at the midpoint of the supporting beam forming acommon joint 36.

Each link spring of the anti-phase couplers consists of a relativelylong, stiff stem 40 and a pair of short flexible beams, or hinges, 38connected to the two ends of the stem. The flexible beams are orientatedsubstantially along the longitudinal direction of the stem. Under thisarrangement, a link spring can be approximated as a rigid bar withrotation hinges attached to its both ends so that it is stiff in itslongitudinal direction while permitting rotation about the z-axis, whichis perpendicular to the x, y reference plane.

In operation, the link springs transfer the drive-mode movement ofmasses along the x-axis to joints 36, which are constrained for linearmovement along the y-axis by supporting beams 34. When the two massesmove in an anti-phase manner, as illustrated in FIG. 2( a), two linksprings in each anti-phase coupler tend to move their joint in samedirection along the y-axis and cause the supporting beam to bend. Sincethe supporting beam is flexible in that direction, the anti-phasemovement of the masses experiences minimal resistance from theanti-phase coupler. With symmetric design about the y-axis, the twomasses have equal amplitudes in drive-mode oscillation, and the phasedifference between the masses is precisely 180 degrees, which isguaranteed by the geometry of the anti-phase coupler and tolerant tofabrication process imperfections.

When the two masses try to move in in-phase manner, as illustrated inFIG. 2( b), two link springs in each anti-phase coupler tend to causetheir joint to move in opposite direction along the y-axis and to canceleach other ideally, which actually results in suppression of thein-phase movement. However, in a real device, the constraints of jointsfrom supporting beams are not ideal. As also shown in FIG. 2( b), joints36 can still have, to some extent, some undesired translational androtational freedoms caused by the asymmetric bending of supporting beams34 and bending or buckling of hinges 38, which leads to overalldeformation of the link springs in their longitudinal directions. As aresult, the in-phase mode of the two masses is not suppressedcompletely. However, the resonant frequency of the in-phase mode in atypical design is still significantly higher than that of the anti-phasemode, which is the working mode of the rate sensor in operation.

With fixed working mode frequency, maximizing the in-phase mode resonantfrequency in a dual-mass gyroscope is important to improve itsperformance of linear vibration noise rejection. In general, the largerthe tilt angle of link spring the higher efficiency in transferring thelongitudinal movement of the masses along the x-axis into lateralmotions along the y-axis at joints 36, which result in stronger couplingeffect of the anti-phase couplers. However, to accommodate larger motionat the joints, supporting beams 34 have to be softer in order to keepthe anti-phase mode frequency constant. The softer supporting beamsresult in weaker constraint of joints in undesired freedoms, such as thetranslation along the x-axis and the rotation about the z-axis, whichactually cause weaker in-phase mode. Therefore, there is a tradeoff inmaximizing the in-phase mode frequency, which depends on detailed designof the gyroscope.

To maximize the in-phase mode frequency under fixed tilt angle of linksprings, it is important to increase the longitudinal stiffness of thelink springs. Lower longitudinal stiffness results in lower efficiencyin transferring the movement of the masses in the x-direction to themotion of the joints in the y-direction. In the worst case, a linkspring can undergo buckling, which leads to zero transferring efficiencyand no coupling effect between the masses at all. Compared to a simplestraight beam, a link spring with a stiff stem in the middle can havesignificantly increased longitudinal stiffness and buckling thresholdthat leads to much higher efficiency in motion transferring.

It is also important to increase the overall linear stiffness of thelink springs along the drive direction so to reduce their deformationunder linear acceleration along the x-axis. In general, the longer andstiffer stems 40, and shorter hinge beams 38 lead to more rigid in-phasemode. However, the shorter hinge beam requires smaller width to keep itflexible enough to serve as a rotation hinge, which increases theprocess sensitivity of its spring constant.

In a typical design, with a hinge length of ⅓-½ of that of stem and tiltangle of 10-30 degrees from the y-axis, the in-phase mode frequency canbe pushed to 3-5 times of that of anti-phase drive-mode, which greatlysuppresses the displacement of the masses in response to linearacceleration along the drive axis.

It should also be noted that the anti-phase coupler structure describedabove could lead to the best performance in suppressing the in-phasemode. FIG. 3 shows another embodiment of a y-axis rate sensor which isvery similar to the embodiment shown in FIG. 1, except it uses straightbeams 48 as link springs. This link spring design can be viewed as aspecial case of that of the embodiment shown in FIG. 1, if the width ofstems 40 in that embodiment is reduced to the same as that of hingebeams 38. With proper design optimization of the tilt angle anddimensions of related beams, fairy good in-phase mode suppression mayalso be achieved.

One should note that the anti-phase coupling design disclosed in thepresent invention has a significant improvement over the prior art, suchas disclosed in U.S. Pat. No. 5,635,638, wherein the coupling member,which is equivalent to link spring of the present invention, is anarcuate member. Due to its curved shape that is prone to buckling andbending in multiple directions, the arcuate member can only be viewed asa poor link spring with significantly lower stiffness in longitudinaldirection and overall linear stiffness along the drive axis than that oflink springs disclosed in the present invention. Therefore, it resultsin much lower efficiency in motion transferring and more displacementunder linear acceleration, which lead to lower efficiency in anti-phasecoupling and in-phase suppression.

FIG. 4 shows an embodiment of a z-axis rate sensor using similaranti-phase coupled dual-mass drive as that in the embodiment of a y-axisrate sensor shown in FIG. 1. Since very similar masses and couplingdesigns are used, they are assigned same numbers as that in the previousembodiment. Again, masses 20 a, 20 b are suspended from sensing frame22, and linked by anti-phase couplers 30, which consist of link springs32 a, 32 b and supporting beams 34, for anti-phase drive-modeoscillation along the x-axis.

However, the sensing frame in this embodiment is suspended by fourflexures 50 disposed at the corners of the rectangular frame andconnected to anchors 52. Each flexure is generally stiff in longitudinaldirection but compliant in its lateral direction in the plane of themasses. The flexures are preferably oriented along radial directionsthat permit the sensing frame to rotate about the z-axis but resistother movements.

In operation, the masses are driven electrostatically by combs 44 a, 44b to oscillate in anti-phase manner along the x-direction. The rotationof masses about the input axis, which is the z-axis, produces Coriolisforces that cause the masses and the sensing frame to rotate about thez-axis. This sense-mode rotation of the sensing frame about the z-axisis monitored by capacitors 54 which have interleaved parallel plates 56,58 connected to the sensing frame and to anchors 60 respectively.

The rate sensor of this embodiment has longitudinal comb drives 44 a, 44b oriented along the drive axis with movable fingers directly attachedto the proof masses for driving the masses to oscillate in drive-mode.One troublesome side effect of this straightforward drive approach isthat small imbalances of the gaps between the fingers induce a lateralmotion in addition to the longitudinal drive-mode oscillation along thex-axis. The imbalances of the gaps of fingers can be caused by processimperfections, stress, or external acceleration, etc, which aredifficult to eliminate completely.

This lateral motion caused by the gap imbalance has a component that isin-phase with the motion caused by the Coriolis forces. Unlikequadrature signal, it cannot be separated and rejected from the realrate signal by phase sensitive circuitry. Furthermore, any instabilityof this in-phase signal directly contributes to the noise and error ofthe output of the rate sensor.

This problem is solved in the present invention by using a decoupledcomb drive system in which the drive motion of the comb fingers isperpendicular to the drive axis of the sensor and decoupled from thesense-mode movement. This drive design is used in another embodiment ofa z-axis rate sensor, as shown in FIG. 5.

This embodiment is generally similar to the embodiment of FIG. 4, exceptit uses a new drive system to replace the comb drives 44 a, 44 b of FIG.4. The drive system is disposed in the center area surrounded by masses20 a, 20 b and anti-phase couplers 30. The drive system consists of twoidentical sub drive systems that are disposed along the y-axis and aresymmetrical about the x-axis.

Each sub drive system is generally symmetric about the y-axis. Itincludes a T-shape rigid driving frame 61, which consists of an arm 62extending along the x-direction and arm 64 extending along the y-axis,and comb drives 66 with moving fingers affixed to arm 62 and stationaryfingers anchored to the substrate. The driving frame is suspended bysuspension flexures 68 which extend along the x-axis and connect toanchors. These flexures permit the driving frame to move along they-axis and severely resist movement in all other directions. The combdrives are oriented with fingers parallel to the y-axis for exciting thedriving frames to oscillate along the y-axis in drive-mode.

Each driving frame 61 is connected to respective joint 36 by a linkflexure 70, which extends along the y-axis. The link flexure isrelatively stiff in its longitudinal direction, but relatively compliantin its lateral direction for bending in the plane of the masses.

In operation, comb drives 66 electrostatically excite each frame 61 tomove in the y-direction. This motion is linked and transferred by linkflexure 70 to joint 36, and further transferred through link springs 32a, 32 b to move the masses along the x-axis. Since the two sub drivesystems are disposed symmetrically about the x-axis, the same drivesignal applied on all combs 66 causes two joints 36 of the sensor tomove in opposite directions along the y-axis. Thus, the two link springsconnected to each mass tend to move the mass in same direction. Again,as in the previous embodiments, the anti-phase couplers 30 connected tothe masses ensure the two masses to oscillate in precise anti-phasemanner with equal amplitudes.

When the rate sensor rotates about its input axis, which is the z-axis,Coriolis forces produced by this rotation and the velocities of themasses of drive-mode oscillation cause the masses and the sensing frameto rotate about the z-axis. Such sense-mode movement experiences littleresistance from link flexures 70, because they are compliant in lateraldirections.

In this embodiment, the erroneous output of the sensor induced by thelateral motion of drive fingers caused by imbalances of the gaps of thefingers is greatly suppressed. First, the lateral movement of drivefingers result from the same gap imbalances is significantly reduced,because driving frames 61 are now severely resistant to movement in thelateral direction by the suspension flexures 68 which extend along thatdirection. Second, link flexures 70 are compliant in this lateraldirection so that the lateral motion transferred to the sensing frame isfurther reduced.

Another advantage of this drive design is that the amplitude of massesin drive-mode oscillation can be different from that of comb drives 66,and can be tuned by adjusting the tilt angle of the link springs. Thisallows one to design a sensor with large drive amplitude of the massesbut relative short fingers of comb drives. This results in moreefficient drive and smaller device. Also interesting is the oppositecase, where the travel distance of comb is larger than the driveamplitude of the masses in drive-mode. The larger travel distance of thecombs can put in more electrostatic energy to the system in each drivecycle, thus the drive voltage can be reduced in order to drive themasses into the same amplitude. This is particularly desirable forgyroscopes operating in atmosphere or poor vacuum environment with low Q(quality factor) that usually require high drive voltages.

As shown in FIG. 5, the two sub drive systems can be further coupledtogether by another pair of anti-phase couplers 72 which are disposedalong the x-axis and symmetric about the y-axis. Each coupler is alsosymmetric about the x-axis. The design of anti-phase coupler 72 issimilar to that of anti-phase coupler 30, each includes a pair of linksprings 74 and a supporting beam 76. The two link springs in eachanti-phase coupler 72 are tilted slightly from the x-axis and connectedrespectively to the ends of rigid arms 64. And the supporting beamswhich extend along the y-axis are connected to the anchors affixed tothe substrate.

Anti-phase couplers 72 further suppress the in-phase movement of the twosub drive systems, and ensure precise anti-phase and equal amplitudes ofthe two sub drive systems in drive-mode. That makes the drive systemimmune to the external acceleration along the y-axis.

For simplicity, each link spring 74 in anti-phase couplers 72 is drawnas a simple straight beam. Certainly, more complicated shape similar tothat of 32 a in anti-phase coupler 30 can be used for betterperformance.

This decoupled drive approach can also be applied to y-axis ratesensors. FIG. 6 illustrates another embodiment of a y-axis gyroscope,which is generally similar to the embodiment of FIG. 1 but employs thesame decoupled drive system shown in the embodiment of FIG. 5.

In this embodiment, the sense-mode motion in response to Coriolis forcesproduced by rate of rotation about the y-axis is the rotation of sensingframe 22 and masses 20 a, 20 b about the y-axis. It should be notedthat, link flexures 70 which extend along the y-axis also permitrotation about the y-axis. Therefore, they impose little resistance tothe sense-mode movement, and decouple the sense motion from the drivesystem.

FIG. 7 shows an embodiment of a dual-axis rate sensor for detecting rateof rotation about the y-, and z-axes. It has similar drive and couplingstructures as that shown in the embodiments of FIG. 5 and FIG. 6, butdifferent sense structure. In addition to sensing frame 22, it hasanother sensing frame 23, which surrounds 22. And frame 22 is suspendedfrom 23 by torsion springs 78 for rotation relative to 23 about they-axis. Frame 23 is suspended from the substrate by a set of flexures 80for rotation about the z-axis.

In operation, masses 20 a, 20 b are driven to oscillate in anti-phasemanner along the x-axis, which is similar to that described in theembodiment of FIG. 5. The rate of rotation about the y-axis producesCoriolis forces that cause the masses and frame 22 to rotate about they-axis. This sense mode rotation is sensed capacitively for monitoringrate of rotation by two electrode plates 42 a, 42 b mounted beneath thetwo masses respectively.

In the meantime, the rate of rotation about the z-axis produces Coriolisforces that cause the masses and frames 22, 23 to rotate together aboutthe z-axis. And this sense-mode rotation is detected for monitoring rateof rotation by parallel-plate capacitors 82, which have interleavedplates with moveable ones mounted on frame 23 and stationary onesaffixed to electrodes anchored on the substrate.

In a typical gyroscope design with comparable resonant frequencies ofthe two sense modes, i.e. rotations about the y-, and z-axis, torsionsprings 78 are often much shorter and substantially stiffer for bendingin the plane than that of flexures 80. Therefore, frames 22 and 23rotate together with little difference in amplitudes in the sense modeof rotation about the z-axis.

Under this design of sense structure, the two sense-mode rotations arewell decoupled from one another, the dual-axis rate sensor monitors twocomponents (y-, and z-) of an angular velocity simultaneously withlittle interference, or cross-talk from one another. Therefore, with twosuch dual-axis sensors rotated 90 degrees about the z-axis from oneanother in same substrate or same package, all three components (x-, y-,and z-) of an angular velocity can be determined simultaneously. This isone way of making a tri-axis rate sensor, which is more efficient andcan have smaller size than conventional approach that assembles threesingle-axis gyroscopes together.

All embodiments of rate sensors described above have sensing framesdisposed outside and surrounding the masses. This approach requiresrelatively larger frame structures that occupy more layout areas. FIG. 8shows another embodiment of a z-axis rate sensor that has a sensingframe surrounded by the masses.

As illustrated in FIG. 8, the rate sensor of this embodiment isessentially symmetric about the x-, and y-axes. Two horseshoe shapedmasses 84 a, 84 b are disposed side-by-side along the x-axis with theopenings of cutout areas facing each other. The masses are suspendedfrom a sensing frame 86, which is disposed in the center area enclosedby the two masses, by suspension flexures 88 extending along the y-axis.The two masses are coupled by a pair of anti-phase couplers 90, each ofwhich consists of a pair of link springs 92 and a supporting beam 94which is mounted to the sensing frame. Link springs 92 are tiltedslightly from the y-direction, and supporting beams 94 extend along thex-axis. The anti-phase couplers ensure the two masses to move inanti-phase manner along the x-axis in drive-mode. Sensing frame 86 issuspended by a set of sense flexures 96 disposed along radial directionsthat permit the frame to rotate about the z-axis and severely resistother movement. These flexures are mounted to the substrate by anchor 98located at the center of the sensor.

In operation, masses 84 a, 84 b are driven electrostatically tooscillate along the x-axis in an anti-phase manner by comb drivers 100a, 100 b respectively. The input rate of rotation about the z-axisproduces Coriolis forces that cause the masses and the sensing frame torotate about the z-axis. This sense-mode rotation is monitored formeasuring rate of rotation by parallel-plate capacitors 102 that haveinterleaved plates mounted to the sensing frame and to anchoredeletrodes.

This embodiment has generally smaller sensing frame, or smaller momentof inertia of sense mode compared to that in the previous embodimentswith sensing frames surrounding the masses. Since the sensing frame is apassive part of the device that serves as a load mass to Coriolisforces, the smaller moment of inertia of the frame leads to greaterCoriolis displacement, thus, higher detection efficiency of the sensor.Also, the sensor of this embodiment has minimal sensitivities totemperature and external stresses, because it employs only one anchorfor suspending the movable structures.

The embodiments described above have anti-phase couplers that are allmounted to the sensing frame and move with the frame in sense-mode.However, rate sensors according to the present invention can also haveanti-phase couplers mounted directly to the substrate.

FIG. 9 illustrates another embodiment of a z-axis gyroscope that hasanti-phase couplers mounted to the substrate. The rate sensor of thisembodiment is symmetric about the x-, and y-axes. Two proof masses 103a, 103 b disposed along the x-axis are suspended from a sensing frame104 by flexures 105, which extend along the y-axis and constrain themasses for movement along the x-axis. Frame 104 is suspended by a set offlexures 106 and constrained for rotation about the z-axis.

Masses 103 a, 103 b are further connected respectively to driving frames107 a, 107 b by flexures 108 which extend along the x-axis. The twodriving frames are disposed side-by-side along the x-axis in the regionbetween the two masses. The driving frames are suspended from thesubstrate by a set of flexures 110 for movement along the x-axis. Combdrives 122 a, 122 b are attached to the driving frames respectively forexciting the frames to oscillate along the x-axis in drive-mode.

The two driving frames are further coupled by a pair of anti-phasecouplers 112, which are disposed spaced apart along the y-axis andsymmetric about the x-axis, for anti-phase movement along the x-axis.Similar to anti-phase coupler 30 of the embodiment of FIG. 1, eachanti-phase coupler 112 includes a pair of tilted link springs 114 and asupporting beam 120. Each link spring 114 consists of a stiff stem 116and two hinge flexures 118. Each supporting beam 120 extends along thex-axis and is affixed to the substrate at its two ends.

In operation, frames 107 a, 107 b are driven by comb drives 122 a, 122 brespectively to oscillate in anti-phase manner along the x-axis indrive-mode. Flexures 108 transfer the movement of the driving frames tothe masses. Since flexures 108 are stiff in longitudinal direction,masses 103 a, 103 b are rigidly linked to frames 107 a, 107 brespectively for movement along the x-axis, the phase of each mass isthe same as that of the driving frame connected to it. Thus, the twomasses are also oscillating in precise anti-phase manner along thex-axis. The rate of rotation about the z-axis produces Coriolis forcesthat cause the masses and the sensing frame to rotate about the z-axis.Since the masses move in lateral directions of flexures 108, theseflexures impose little resistance to the sense-mode rotation. Thisrotation movement is sensed capacitively by the parallel-platecapacitors 124, which have interleaved plates attached to the sensingframe and to the anchors affixed to the substrate.

Unlike the embodiment of FIG. 5, the drive directions of the comb drivesin this embodiment are parallel to the drive axis, i.e. the x-axis. Andthe driving frames serve as both the decoupled driver and part ofanti-phase coupling means for the two masses. This is more efficientthan that of the embodiment of FIG. 5, wherein the masses haveadditional anti-phase couplers 30 that directly couple the masses.However, this approach cannot be directly applied in y-axis gyroscope,because link flexures 108 resist the sense-mode rotation about they-axis.

The rate sensors disclosed in the previous embodiments all have a singlesensing frame for each sense-mode. FIG. 10 shows another embodiment of az-axis rate sensor that has dual sensing frames for the sense-mode. Inthis embodiment, two masses 126 a, 126 b, which are disposed along thex-axis, are suspended from two driving frames 128 a, 128 b respectivelyby flexures 130 that extend along the x-axis. Flexures 130 link eachmass to move together with its corresponding driving frame along thex-axis, while permit it to move relative to the frame along the y-axis,which is the sense axis of the mass.

Driving frames 128 a, 128 b are disposed side-by-side in a regionbetween the two masses, and suspended by flexures 132 which extend alongthe y-axis and constrain the frames for movement along the x-axis. Thetwo driving frames are coupled by a pair of anti-phase coupler 134,which are disposed along the y-axis and constrain the driving frames foranti-phase movement along the x-axis. Each anti-phase coupler 134consists of a pair of tilted link springs 136 and a supporting beam 138which extend along the x-axis and mounted to the substrate.

Each mass is connected to a pair of sensing frames 142 by flexures 140which extend along the y-axis. The two sensing frames extending alongthe x-axis are disposed spaced apart along the y-axis. Each sensingframe is suspended by a flexure 144 which extends along the y-axis andare anchored to the substrate, and a flexure 146 that extend along thex-axis with its center affixed to the substrate. These two flexuresconstrain the sensing frame for rotation about an axis that is parallelto the z-axis and passes through the interception point of theextrapolating of the two flexures.

Masses 126 a, 126 b are further connected to two additional drivingframes 150 a, 150 b respectively by flexures 151 extending along thex-axis. These frames are suspended from the substrate by flexures 152extending along the y-axis. With these two additional frames, the massescan be well constrained for drive-mode movement along the x-axis withthe driving frames, and sense-mode movement along the y-axis relative tothe driving frames. And comb drives 154 attached to frames 150 a, 150 bcan be used to drive the masses to oscillate in drive-mode or monitoringthe velocity of the masses for drive loop control.

In operation, the masses are driven to oscillate in anti-phase manneralong the x-axis. The Coriolis forces produced by the rate of rotationabout the z-axis cause the masses to move in their sense directionsparallel to the y-axis. This sense movement is transferred to twosensing frames 142 by flexures 140 to cause each sensing frame to rotateabout its rotation axis parallel to the z-axis. The rotations of the twosensing frames are in-phase to one another. And this sense-mode rotationis monitored by parallel-plate capacitors 148 attached to the frames formeasuring rate of rotation.

The use of dual rotation sensing frames in this embodiment reduces theinertia moment of inactive parts in the sense mode, in comparison to thesingle sensing frame used in the previous embodiments. Thus, it improvesthe detection efficiency of the rate sensor.

FIG. 11 shows another embodiment of a y-axis rate sensor which hassimilar structure as to the embodiment of FIG. 6, but has differentoperation mode with reversed drive and sense functionalities compared tothat of FIG. 6. The drive signals are now applied to electrode plates 42a, 42 b to drive the masses 20 a, 20 b and supporting frame 22, whichserves as a driving frame now, to move out-of-plane and rotate about they-axis. In presence of rate of rotation about the y-axis, Coriolisforces produced by this rotation and the velocities of the masses inz-axis cause the masses to move in anti-phase manner along the x-axis,which is now the sense axis of the masses. This movement is transferredto joints 36 by link springs 32 a, 32 b for movement along the y-axis.The movement of joints 36 is further transferred by link flexures 70 toa pair of sensing frames 156, which are spaced apart along the y-axis,which is a normal axis perpendicular to the sense axis. The sense framesare constrained for movement primary along the y-axis by a set offlexures 158 which extend along the x-axis.

The two sensing frames are coupled by anti-phase couplers 72 to move inanti-phase manner along the y-axis, and to further suppress theirin-phase movement. The anti-phase movement of the sensing frames ismonitored for measuring rate of rotation about the y-axis byparallel-plate capacitors 160 which have interleaved plates attached tothe sensing frames and connected to the anchors affixed to thesubstrate.

It can be seen, driving frames 64 of the embodiment of FIG. 6 arereplaced by sensing frames 156, and drive combs 66 are replaced byparallel-plate capacitors 160 in this embodiment. The combination ofanti-phase couplers 30 and 72 ensure the masses, as well as sensingframes to oscillate in anti-phase manners, so that the sense-mode isseverely resistant to external linear acceleration along the x- andy-axes. This leads to superior performance of the sensor in vibrationnoise rejection.

It should be noted that, in this embodiment, the sense-mode motion ofthe sensing frames can be larger than that of the masses by adjustingthe tilt angle of the link springs, which provides a mechanicalamplifier for the sense mode. Amplified sensing frame displacement leadsto larger capacitance output for same angular rate input, which resultsin higher sensitivity of the sensor.

Similar modifications can also be made to the embodiment of thedual-axis rate sensor illustrated in FIG. 7, so that it becomes adual-axis gyroscope for measuring rate of rotation about the x-, andy-axes, as shown in FIG. 12. This embodiment of dual-axis rate sensorcan be viewed as a y-axis rate sensor, as shown in FIG. 11, suspended bya pair of torsion springs 26 from sensing frame 23, which is mounted forrotation about the z-axis. Torsion springs 26 permit driving frame 22 torotate about the y-axis relative to sensing frame 23 while severelyresisting other relative movement between the two frames. Sensing frame23 adds an additional rotation freedom to the device that enables themasses to rotate about the z-axis, in addition to the x-axis.

Again, masses 20 a, 20 b are driven electrostatically by the electrodes42 a, 42 b respectively to rotate with frame 22 about the y-axis indrive-mode. The Coriolis forces produced by the velocities of the massesin the z-axis and rate of rotation about the y-axis cause the masses tomove along the x-axis. The movements of the masses along the x-axis arelinked and transferred to sensing frames 156 for oscillation along they-axis in anti-phase manner. This sense-mode movement is monitored bycapacitors 160 for measuring the rate of rotation about the y-axis. Inthe meantime, the Coriolis forces produced by the velocities of themasses in the z-axis and rate of rotation about the x-axis cause themasses and frames 22, 23 to rotate together about the z-axis. Thesense-mode rotation of frame 23 is monitored by parallel-platecapacitors 82 for measuring rate of rotation about the x-axis.

Since the movements of the masses in the two sense modes are independentfrom one another, i.e. the two sense modes are decoupled, this dual-axisrate sensor can measure the two components (x-, and y-) of an angularvelocity simultaneously with minimal interference or cross-talk.

With similar modifications, the embodiment of FIG. 5 can also haveanother operation mode by switching the drive and sense functionalities.As shown in FIG. 13, a driving frame 162 suspended for rotation aboutthe z-axis replaces sensing frame 22 of FIG. 5. Comb drivers 164 a, 164b, which have movable fingers attached to the driving frame, aredisposed along the circumference of the frame for driving the frame torotate about the z-axis. Since comb drivers 164 a and 164 b in each pairare oriented in opposite directions, they can be used for differentialdrive, or one for drive and the other for drive amplitude monitoring.

Since flexures 24 a, 24 b, which suspend the masses from the drivingframe, are stiff in longitudinal directions, the masses move togetherwith the driving frame to rotate about the z-axis in drive-mode. Inpresence of rate of rotation about the z-axis, Coriolis forces producedby this rotation and the velocities of the masses along the y-axis causethe masses to move along their sense axis, i.e. the x-axis, inanti-phase manner. Similar to that in the embodiment of FIG. 11, themovement of the masses are linked and transferred to a pair of coupledsensing frames 156 for anti-phase movement along the y-axis. Thismovement is sensed by capacitors 160 for measuring the rate of rotationabout the z-axis.

FIG. 14 shows another embodiment of a z-axis gyroscope that is modifiedfrom the embodiment of FIG. 9 with switched drive and sensefunctionalities. A driving frame 166 which is mounted for rotation aboutthe z-axis and driven by comb drives 167 a, 167 b replaces sensing frame104 in FIG. 9.

Similar to the embodiment of FIG. 13, masses 103 a, 103 b move withframe 166 for rotation about the z-axis in drive-mode. In presence ofrate of rotation about the z-axis, Coriolis forces produced by thisrotation and the velocities of the masses in the y-direction cause themasses to move along the x-axis in anti-phase manner. The movements ofthe masses are transferred by flexures 108 to a pair of sensing frames168 which are constrained for anti-phase movement along the x-axis. Thissense-mode movement of the sensing frames is monitored by the capacitors170 for measuring rate of rotation about the z-axis. The drive approachshown as in FIG. 13 and FIG. 14 can also be applied to the embodiment ofdual-axis gyroscope of FIG. 12 so that it becomes a dual-axis gyroscopefor measuring the rate of rotation about the x-, and z-axes, as shown inFIG. 15. A driving frame 172 in this embodiment replaces sensing frame23 of FIG. 12. The driving frame is driven by comb drives 173 a, 173 bto rotate with supporting frame 22 and masses 20 a, 20 b about thez-axis in drive-mode.

In presence of rate of rotation about the x-axis, Coriolis forcesproduced by this rotation and the velocities of the masses along they-axis cause the masses and frame 22 to rotate about the y-axis. Thissense-mode movement is detected capacitively by electrode plates 42 a,42 b which are placed underneath the masses for measuring rate ofrotation about the x-axis.

In presence of angular rate input about the z-axis, Coriolis forcesproduced by that rotation and the velocities of the masses along they-axis cause the masses to move along the x-axis in anti-phase manner.This movement is transferred to sensing frames 156 for anti-phasemovement along the y-axis. This sense-mode movement of the sensingframes is monitored by parallel-plate capacitors 160 for measuring rateof rotation about the z-axis.

FIG. 16 shows an embodiment of a tri-axis gyroscope for measuring rateof rotations about the x-, y-, and z-axes. It employs two sub-sensors,each of which is a dual-axis rate sensor as shown in FIG. 15. The twosub-sensors are disposed side-by-side along the x-axis with the rightone rotated 90 degrees about the z-axis from the left one. The leftsub-sensor measures rate of rotation about the x-, and z-axes, and theright sub-sensor measures rate of rotation about the y-, and z-axes.

The two sub-sensors can be further coupled together by a coupling spring174, which connect the midpoints of the adjacent edges of driving frames172 a, 172 b of the two sub-sensors. The coupling spring is in the formof a small rectangular frame, with relative long, flexible flexuresextending in the y-direction and relatively short, stiff ones in thex-direction. This spring constrains frames 172 a, 172 b to rotate inanti-phase manner about their drive axes 176 a, 176 b, which locate atthe centers of the frames respectively and extend parallel to thez-axis.

The structures of the two sub-sensors can be identical, so that theiroscillations about the drive axes in drive-mode are equal in amplitudeand precisely anti-phase to one another. Therefore, the net angulardrive momentum is zero and the device does not inject vibration energyinto the substrate.

Since the overall tri-axis rate sensor has a single resonant mode thatis excited as drive-mode, a single drive loop is sufficient to achieveamplitude-controlled drive-mode oscillation of all masses in the sensor.It simplifies the circuitry of the device and lowers cost.

The tri-axis rate sensor of this embodiment includes two rate sensorsfor measuring rate of rotation about the z-axis. That could beadvantageous for some applications, such as stability controlling forautomobile, which is desirable to have a redundant YAW rate sensor.

FIG. 17 illustrates another embodiment of a tri-axis rate sensor whichemploys two subsensors without redundant sensors about the z-axis. Theleft subsensor is similar to that of the previous embodiment formeasuring rate of rotation about the x-, and z-axes. However, the rightsubsensor is simplified to a single axis rate sensor for measuring rateof rotation about the y-axis only.

The right subsensor includes a driving frame 178 mounted for rotationabout a rotation axis, or driving axis, 177, which is parallel to thez-axis. Mass 180 is disposed within the driving frame and suspended fromit by a pair of torsion springs 182, which are spared apart and extendalong the x-axis. The torsion springs permit the mass to rotate aboutthe x-axis and severely resist other relative movements between the massand the driving frame.

In operation, driving frame 178 and mass 180 are excited to rotate aboutdriving axis 177 in drive-mode by comb drives 186 a, 186 b, which havemoving comb fingers attached to driving frame 178 and stationary fingersconnected to anchored electrodes. Coriolis forces produced by rate ofrotation about the y-axis cause the mass to rotate about the x-axis.This sense-mode rotation movement is detected capacitively by electrodeplates 184 a, 184 b, which are disposed underneath the mass and spacedapart along the y-axis, for monitoring rate of rotation about they-axis.

Similar to the embodiment of FIG. 16, the two subsensors can be furthercoupled together to have a common drive-mode by a coupling spring 188,which is similar to 174 in the previous embodiment and are connected tothe midpoints of adjacent edges of driving frames 172 a and 178.

The rate sensors according the present invention can be made from amaterial such as single-crystal silicon, polycrystalline silicon, metal,or other conductive materials, or insulate materials coated withconductive films, on a substrate such as silicon, glass, or othermaterials, by suitable MEMS process such as deep-reactive-ion-etching.And the sensor may be operated in atmosphere ambient or in vacuumhousing for better performances.

It is apparent from the foregoing that a new and improved angular ratesensor has been provided. While only presently preferred embodimentshave been described in detail, as will be apparent to those familiarwith the art, certain changes and modifications can be made withoutdeparting from the scope of the invention as defined by the followingclaims.

1. An angular rate sensor comprising: a planar substrate; first andsecond generally planar masses disposed above the substrate and spacedapart along a pre-selected drive axis in the plane of the masses; aninput axis perpendicular to the plane of the masses; a sense axis in theplane of the masses and perpendicular to the drive axis; first andsecond driving frames disposed along the drive axis in a region betweenthe masses; coupling means for constraining the driving frames foranti-phase movement along the drive axis; flexures connecting the firstand second masses to the first and second driving frames respectively,wherein said flexures substantially constrain the masses to movetogether with respective driving frames along the drive axis indrive-mode, and permit the masses to move relatively to the drivingframes in directions parallel to the sense axis in sense-mode; actuationmeans for exciting the driving frames to oscillate along the drive axisin drive-mode; detection means responsive to the anti-phase movement ofthe masses in directions parallel to the sense axis for monitoring rateof rotation about the input axis.
 2. The rate sensor of claim 1 whereinsaid coupling means includes a pair of anti-phase couplers disposed in aregion between the drive frames and spaced apart along the sense axis,each said anti-phase coupler comprising: a supporting beam which extendsin a direction substantially parallel to the drive axis and is mountedto the substrate at its two ends; first and second link springs whichextend from the first and second driving frames respectively to thesupporting beam and join one another to form a common joint atsubstantially the midpoint of the supporting beam.
 3. The rate sensor ofclaim 2 wherein each said link spring is a straight beam.
 4. The ratesensor of claim 2 wherein each said link spring comprises: asubstantially stiff stem; first and second hinges connected to the twoends of said stem respectively, wherein said hinges are flexible beamspermitting bending in the plane of the masses.
 5. The rate sensor ofclaim 2 wherein said link springs are tilted from a direction parallelto the sense axis at a predetermined angle.
 6. The rate sensor of claim1 wherein said actuation means includes comb drives attached to at leastone of the driving frames.
 7. The rate sensor of claim 1 wherein saiddetection means comprising: a sensing frame disposed in the plane of themasses, wherein said sensing frame surrounds the masses; flexuresmounting the masses on the sensing frame and constraining the masses formovement relative to the sensing frame along the drive axis; flexturessuspending the sensing frame from the substrate for rotation about theinput axis in response to Coriolis forces produced by rotation of massesabout the input axis; capacitors with movable electrodes attached to thesensing frame for detecting the rotation movement of the sensing frameabout the input axis for monitoring rate of rotation about the inputaxis.
 8. The rate sensor of claim 1 wherein said detection meanscomprising: first and second sensing frames disposed along the senseaxis with the masses located between the sensing frames, wherein saidsensing frames extend in directions parallel to the drive axis; meansfor suspending the sensing frames for torsional movement aboutrespective rotation axes which are parallel to the input axis and spacedapart along the sense axis; flexures extending in directions parallel tothe sense axis and connecting the masses to the sensing frames fortransferring anti-phase movement of the masses in directions parallel tothe sense axis to rotation movement of the sensing frames about theirrotation axes; actuation means for exciting the driving frames tooscillate along the drive axis in drive-mode; capacitors with movableelectrodes attached to the sensing frames for detecting the rotationmovement of the sensing frames about their rotation axes for monitoringrate of rotation about the input axis.
 9. The rate sensor of claim 8wherein means for suspending each sensing frame including: a firstflexible beam extended parallel to the drive axis with its two endsmounted to the sensing frame and midpoint anchored to the substrate; asecond flexible beam extended along the sense axis with one endconnected to the sensing frame and the other end anchored to thesubstrate.